METHOD AND DEVICE FOR CONTINUOUS MONITORING AND ALIGNMENT OF THE PROPELLER SHAFT OF A SHIP

Abstract: The invention relates to a method and a device for continuous and automatic monitoring and alignment of a propeller shaft (A), mounted in a plurality of bearings, in a ship upon hull deformations during operation. The inventive method substantially involves: measuring a parameter indicating the state of stresses in the shaft (A); calculating, by means of a special computer program, how the bearings (L) (the offsets) should be changed in view of the measured parameter and the prescribed conditions; adjusting the respective offsets by the calculated change. The inventive device substantially consists of: measuring means (M) which monitor a parameter indicating the state of stresses in the shaft (A); a computer (D) which receives values from the measuring means (M) and calculates, by means of a special program, how the bearings (the offsets) should be changed; control means (S) which receive the offset changes from the computer (D); an actuator which is actuated by the control means (S) and adjusts the respective offsets by the calculated change.

Description: METHOD AND DEVICE FOR CONTINUOUS MONITORING AND ALIGNMENT OF THE PROPELLER SHAFT OF A SHIP The present invention relates to a method and a device for continuous and automatic monitoring and align¬ ment of the propeller shaft of a ship in operation. A propeller shaft should transmit a torque from the engine to the propeller. The shaft is supported by a num¬ ber of bearings. Static and dynamic loads are taken up in the vertical and horizontal planes. The deflection of the shaft (shaft line) determines the load distribution between the bearings as well as the bending stresses in the shaft. Static bearing loads (the shaft weight) and bending stresses are balanced by affecting the deflection by alignment of the shaft. Such an alignment is carried out in connection with the installation or repairing of the shaft. Three or more bearings are required. The alignment is achieved by elevating or depressing different bearings a few millimetres at the most, in relation to a horizontal reference line, with an accuracy of a few tenths of a millimetre. The reference line is related to the stern tube bearing. The amount by which the different bearings should be elevated or depressed to obtain a certain load F t on bearing is calculated according to the equation system: F l = F i + a U x l + a 12 x 2- a ln x n ,- , ••• Q UJ F n = F n + a nl x l + a n2 x 2 ' " a nn x n n : the number of bearings / : a bearing in the succession of bearings from the aft (i = 1,2, ...,«) a jj : the change of the load on bearing when bearing j is elevated 1 mm x. : the elevating or depressing (offset) of bearing i F. : the load on bearing when all x = 0. Computer programs are used for calculating a (in- fluence matrix) and solving the equation system (1). The solution is obtained by "trial and error" with different offsets x as input data. If all the bearings have their centres on the refe¬ rence line (all x = 0), the distribution of the load between the different bearings becomes most unfavourable. The stern tube bearing is excessively loaded because of the weight of the propeller. The other bearings run the risk of becoming over- or underloaded. Moreover, there is a risk of detrimental stresses occurring in the shaft. In addition to static forces, the bearings should take up dynamic forces produced by bending vibrations during the rotation of the shaft. Such dynamic forces increase to a serious extent if resonance occurs between one vibration frequency and one of the lower natural fre- quencies of the shaft. After the propeller shaft has been installed, it is desirable to avoid major hull deformations. It is there¬ fore customary to mount the superstructure on the hull before installing the shaft. However, with the super- structure mounted, it becomes more difficult to mount the shaft. The propeller shaft is installed in sections. Each section is placed on two bearings. To obtain the calcu¬ lated offsets and the required shaft line, each bearing is elevated or depressed by means of jacks and adjusting screws. The bearing position is checked with the aid of sophisticated measuring equipment. When a bearing is in the correct position, spacer plates are inserted between the bottom plate of the bear- ing and the foundation, whereupon the bearing is fixed. The spacer plates are specially ground for each bearing. The plate thickness should give the bearing the desired offset. Below are reported some patents which are concern¬ ed with the alignment of propeller shafts in ships. From EP-A-015 654 is known a method for predeter¬ mining the position of the parts in a structure before mounting the parts. With the aid of optical instruments provided with sights, the positions of abutments are established in relation to reference surfaces. This method can be used for obtaining the correct inclination of a vessel engine. From WO87/01439 is known a device for controlling the static position of a shaft in a certain point. A light beam is reflected by a prism fixed on the shaft towards a receiver. In the receiver, the light beam gene¬ rates an electric current, the strength of which depends on the angle of incidence. The current intensity is com¬ pared with a desired value. From EP-A2-405 777 are known a method and a device for measuring a shaft line. A laser beam is directed at at least two sensors fixed on the shaft. By comparing signals generated in the sensors, deviations from the initial shaft line are determined. The patent publications mentioned above relate to different methods or devices for providing a satisfactory alignment with fixed bearings in connection with instal¬ lation or repairing. As far as we know, this also applies to other patents relating to aligning techniques. When calculating the shaft line, one takes into account imagined conditions during operation and stand¬ still, hot and cold states, respectively. Generally, the offsets are determined such that the bearing loads are distributed as favourably as possible in the hot state. It is checked that acceptable conditions prevail in the cold state. A real and a calculated shaft line seldom agree. Since the bearings are fixed in the foundation, the off- sets and the shaft line are affected by hull deformations due to sea, load, heat etc. In medium-size and large-size vessels, the bearings can be elevated or depressed some centimetres. Since an alignment is carried out with an accuracy of tenths of a millimetre, it is understood that hull deformations give rise to under- and overloads in the bearings. This means wear to the bearings. Unevenly loaded gear bearings cause wear to the gear teeth. In underloaded bearings, "oil whip" (instability) occurs. Realignment is sometimes necessary after a cer¬ tain time of seaway. For example, this is so if the hull has sagged or if some other influencing factors deviate considerably from what was assumed in the designing. Realignment after launching is however rendered more dif¬ ficult by the fact that certain bearings are built in. One object of the present invention is to provide a method for continuous and automatic monitoring and align¬ ment of the propeller shaft of a ship upon hull deforma- tions during operation. Another object of the invention is to maintain an optimum shaft line. "Optimum shaft line" here means a maximally straight shaft line with respect to the pre¬ scribed lower and upper limits for bearing pressure, shaft stresses etc. A third object of the present invention is to pro¬ vide a device for carrying out the inventive method. The method and the device according to the present invention are achieved as recited in claims 1 and 4, respectively. Special embodiments are stated in the sub- claims. The invention confers the following advantages: • the preparatory work for aligning the propeller shaft is facilitated; • limits are established instead of specific values; • hypothetical input data are not required; • the installation of the shaft is simplified; • the superstructure can be mounted on the hull after the installation of the shaft; • the bearing pressures remain within prescribed limits; • the pressures on gear transmission or engine bear¬ ings remain almost constant; • realignment after launching is not required; • extreme bending stresses are avoided; • fatigue is reduced; • the wear to the bearings is reduced; • the noise level is reduced; • bending vibrations are damped; • oil whip in bearings is avoided; • detrimental natural frequencies can be avoided. Fig. 1 shows a shaft line. Fig. 2 shows spacer plates between bearings and foundation. Fig. 3 shows a propeller shaft supported by a num- ber of bearings on piston and cylinder assemblies in hydraulic pressure circuits having a common computer. Fig. 4 shows a bearing on piston and cylinder assem¬ blies having a pressure circuit. An exemplifying embodiment of the present invention will now be described with reference to Figs 1-4. The device according to the invention is intended to operate in the engine room of a ship in operation. By means of the device, the bearing offsets should be adjusted in a manner to compensate for hull deformations. With reference to Figs 3 and 4, the device consists of a piston and cylinder assembly (K) in a hydraulic pres¬ sure circuit having a measuring means (M) , a pump (P) and a control means (S) . The measuring and control means are connected to a common computer (D) . The piston and cylin- der assembly acts between the foundation and the bottom plate of a bearing (L) for a shaft (A) . The bearing load (pressure) is given by the pressure in the pressure circuit. The pressure is changed upon a hull deformation where the bearing is vertically offset. The new pressure p™ 20 is indicated by the measuring means and is supplied to the computer, in which a comparison with desired values is performed. In the event of excessive deviation, a volumetric change AV or a corresponding value is calculated for adjusting the piston. This value is supplied to the con- trol means which starts the pump. The piston is adjusted Δx so as to compensate for the hull deformation. Normally, all the pistons in the system are adjusted at the same time. The calculation requires all the bear¬ ing pressures. Referring to Fig. 3, the process takes place in the following steps: 1. Continuously measuring the pressures p™ ea by the mea¬ suring means (M ) and supplying the values to the computer (D) . 2. Checking in the computer (D) whether p " ≤ p™ a ≤ p™ . If this condition is not satisfied, the following calculations are made: • pressure deviations Δ, = p° pl - p ea ; • offset changes Δx, divided by (1): AP/4 = a tι *ι + a t2^2 ■ • • a i- n , where A Pi = (F° pt - F™ a ) / A, and Δx,. = x - x™ a • volumetric changes AV t yielding piston changes Δx,. 3. Feeding ΔV t or corresponding values to the control means (S t ) . 4. Using the control means (Sj) for starting the pumps (P,) so as to adjust Δx, the pistons (Kf) . In the inventive method, use is made of a computer program which differs essentially from other correspond¬ ing programs. The program includes: • calculation of the optimum offsets (optimum shaft line); • calculation of the shaft line concerned with the aid of prevailing bearing pressures; • calculation of how the offsets should be adjusted to restore the optimum shaft line. The program gives bearing loads and offsets as a result of an optimisation procedure with respect to permissible ranges for influencing parameters. The optimisation pro¬ cedure takes place with a target function: /=|x ; .|+|x 2 |~-|x„|. (2) By the function (2), the sum of the offsets is minimised with respect to subconditions setting lower and upper limits, e.g. for: • bearing pressure; • stresses and deflections in exposed points on the shaft; • the load on the engine flange (indicated by the engine manufacturer), • the difference between the load on the two gear bearings (these bearings should be equally loaded with respect to the tooth engagements). The minimisation of function (2) with respect to the subconditions results in an optimum offset x° pt . Tn is gives a maximally straight shaft line, i.e. an optimum shaft line. After insertion of the optimum offset xf pt in the equation system (1), the load F° pt is obtained on bearing i at an optimum shaft line. The corresponding pressure p° pt = , where A f is the supporting bearing surface. It is assumed that the pressures on n - 2 bearings are available. The possibilities of maintaining an optimum shaft line upon hull deformations depend on how many bearings are adjustable. If n - 2 bearings are adjustable, the same shaft line can be constantly maintained. If less than n - 2 bearings are adjustable, a new shaft line is calculated if the foundation is so changed that the old shaft line cannot be maintained or is no longer optimal. The new shaft line gives a new value of the function (2) depending on the subconditions and the number of adjustable bearings.

Claims: CLAIMS 1. A method for continuous and automatic monitoring and alignment of a propeller shaft (A), mounted in a plu¬ rality of bearings, in a ship upon hull deformations during operation, c h a r a c t e r i s e d by the steps of • measuring a parameter indicating the state of stresses in the shaft (A); • calculating by means of a special computer pro¬ gram how the bearings (L) (the offsets) should be changed in view of the measured parameter and the prescribed conditions; • adjusting the respective offsets by the calculated change. 2. A method as claimed in claim 1, c h a r a c ¬ t e r i s e d by the step of • measuring the pressures exerted on the bearings. 3. A method as claimed in claim 1 or 2, c h a r ¬ a c t e r i s e d by the steps of • feeding the measured pressures to a computer (D), which calculates offset changes by means of a spe¬ cial program; • feeding each offset change to the respective con¬ trol means (S); • actuating, with the aid of each control means (S), an actuator adjusting the respective offsets by the calculated change. 4. A device for continuous and automatic monitor¬ ing and alignment of a propeller shaft (A), mounted in a plurality of bearings, in a ship in operation, c h a r a c t e r i s e d by • measuring means (M) which monitor a parameter indi- eating the state of stresses in the shaft (A) • a computer (D) which receives values from the meas¬ uring means (M) and calculates, by means of a spe- cial program, how the bearings (the offsets) should be changed; • control means (S) which receive the offset changes from the computer (D) • actuators which are actuated by the control means (S) and adjust the respective offsets by the calculated change. 5. A device as claimed in claim 4, c h a r a c ¬ t e r i s e d in that • the measuring means (M) consists of a pressure gauge; • that the actuator consists of a hydraulic cir¬ cuit including, inter alia, a pump (P) and a piston and cylinder assembly (K) which is arranged between the foundation (F) and the bearings (L) .

Inventors: RUBIN ANDERS, RUBIN, ANDERS
Applicants: RUBIN ANDERS, RUBIN, ANDERS
Application year: 1994
Main classification: B63H23
IPC codes: B63H5, B63H23, G01L5